1. Field of the Invention
This invention relates to monolithically integrated semiconductor laser amplifiers and to Master Oscillator-Power Amplifier (MOPA) lasers that have a flared amplifier section (such as MFA-MOPA's). In particular, the invention relates to the optimization of the flare of such amplifiers and MOPA devices to achieve higher output powers.
2. Description of the Related Art
Monolithically integrated semiconductor lasers that have a flared amplifier section are well known and have been extensively studied. For example O'Brien, et al., in IEEE Journal of Quantum Electronics, pages 2052-2057 (1993), study the optical properties of a MFA-MOPA semiconductor laser. This design has allowed higher output powers to be achieved in a single diffraction-limited beam. The paper includes a study of output power, efficiency, spectral quality, virtual source stability (astigmatism), and beam quality.
In U.S. Pat. No. 5,602,864, Welch et al. describe a number of semiconductor laser configurations incorporating a flared amplifier section. The semiconductor active medium is an electrically pumped light amplifying diode heterostructure or "amplifier chip" that has a flared gain region with a narrow, single mode, optical aperture end and a broad light output end. The flared gain region is differentially pumped to ensure high power amplification of forward propagating light while maintaining a single spatial mode of oscillation. The flared region is linearly flared and increases in width toward the output facet of the amplifier chip at a rate that is slightly greater than the divergence of light propagating within the flared gain region.
In U.S. Pat. No. 5,539,571, Welch et al. describe an optical amplifier which is differentially pumped. The differential pumping is applied to a semiconductor amplifier which has a gain region that increases in width toward its output at a rate that equals or exceeds the divergence of the light propagating within the amplifier. Because of the flare, the amplifier is pumped with a reduced current density at its input end relative to its output end. The optical signal is amplified along the length of the flared amplifier such that the peak intensity remains nearly constant and the increase in power is a result of the expanding mode width of the amplifier. By having a low current density at the input end, the injected light beam is first allowed to diverge in the amplifier before receiving strong amplification in the more heavily pumped output region of the amplifier. Hot spots near the input end, which otherwise distort the beam and lead to optical filamentation and a spatially incoherent and multimode output, are thus avoided by providing only a low level of pumping near the input end. Also, by only allowing strong amplification at the output end of the amplifier, spatial hole burning in the input region is avoided. The flare of the pumped gain region of the amplifier is typically linear.
In summary, inventors have clearly demonstrated the benefits of a semiconductor laser amplifier which has a linearly-flared gain region, expanding in width from the input end to the output end of the amplifier. Improvements to the flared configuration have been achieved by differentially pumping the gain region.
However, the output power of the flared semiconductor laser is still limited by the onset of filamentation within the amplifier and the consequential degradation of the laser beam. Accordingly, there is still a need for an optimal approach to the design of laser amplifiers to maximize output power without filamentation within the amplifier.